1. Fields of the Invention
The present invention is directed to a fire alarm system, and more particularly to a fire alarm system in which fire detecting terminal means are connected to a common transmission line to a receiver where the information transmitted from the fire detecting terminal means is processed for determination of fire presence.
2. Description of the Prior Art
Such a fire alarm system is already known in the art as disclosed in U.S. Pat. No. 4,556,873 issued on Dec. 3, 1986 and assigned to the same assignee of this application. This patent utilizes intelligent-type smoke detectors connected to a receiver or central unit through a common signal transmission line comprising two wires. The intelligent-type smoke detector includes a basic function of transmitting a binary information of the sensed smoke density to the receiver in answer to the instruction from the receiver for determination of fire presence on the side of the receiver. Additionally included in the smoke detector as a safeguard against possible failure of transmitting the binary information of the smoke density is a back-up function of providing a level-shifted signal to the receiver over the transmission line in the event that the analog value of the sensed smoke density is determined on the side of the detector to be higher than a predetermined threshold value, which occurrence being acknowledged by the receiver as indicating fire presence independently of the above basic function. The idea behind the above fire alarm system is to provide a back-up operation of successfully monitoring the presence or absence of fire even when the binary information of the sensed smoke density fails to be transmitted to the receiver due to unexpected failure of transmitting the binary information of the sensed smoke density. In fact, the level-shifted signal transmission network is less likely to fail than the digital signal transmission network utilizing a more sophisticated hardware like a CPU and thus can well stands for the back-up operation.
For implementation of the above fire detecting system, it is a normal practice to constantly actuate the digital signal transmission network as a main fire detection scheme for more precise and convenient analysis of fire presence in accordance with the differing environmental conditions of locations to be monitored while disabling the level signal transmission network or back-up fire detection scheme, and set the latter network into operation only when the sensed quantity becomes significantly higher above the threshold level so that it can detect fire presence even in case of the failure of the digital signal transmission network. With this methodology, the level signal transmission network is limited to have a less sensitivity against possible fires than the digital signal transmission network, otherwise the back-up scheme would become operative while the main scheme is in operation so as to nullify or detract from the precise analysis of fire presence even the digital transmission scheme is operating correctly, thus unduly reducing the sensitivity against possible fires.
In this sense, the prior art system is not completely satisfactory in providing a true back-up protection retaining a higher sensitivity substantially equal to the main fire detection scheme so long as the level signal transmission network is rendered inoperative unless there detected a higher sensor output than required by the digital signal transmission network in determining fire presence. Therefore, it is mostly desired for providing the true back-up protection of the fire alarm system which includes the level signal transmission network having the same sensitivity as the main fire detection or digital signal transmission network, although they operate on the different modes of fire detection.